Fine adjustment mechanisms for geodetic instruments, such as theodolites and transits, in the past have generally comprised a pair of control elements, such as knobs or levers, mounted on the instrument to separately effect a locking of alidade or telescope axle bearing means and a driving of the alidate or telescope through small angles of rotation by the application of a thrusting force directed tangentially of the rotational path of those elements of the instrument. Typical of this type of mechanism are those to be observed in U.S. Pat. No. 2,132,170. The disadvantage of such arrangements of separate locking and tangent screw drive control means is apparent in the requirement for the operator to interrupt his target observation in order to locate the appropriate control member for the desired adjustment.
More recently, locking and fine adjustment controls have been combined into coaxial mounting which to some extent has obviated the clumsy nature of the separate control elements. Such previous coaxial arrangements have, however, been particularly hampered in their utility by the requirement for at least one of the control fixtures, usually the locking mechanism, to comprise an extended mechanical drive train. Such a mechanism was thus limited in for additional adjustment of lines of sight, usually through means for shifting the telescope bearing seating, it has been a common requirement that locking and fine adjustment mechanisms afford a substantial degree of flexibility and freedom of movement in order to accommodate such bearing adjustments. Mechanical drive trains in previous geodetic instruments have lacked the necessary combination of flexibility while retaining quick and positive response to applied locking and fine adjust motions.
A further disadvantage in earlier telescope bearing adjustment means results from the common use of shaft eccentric motions which, of necessity, impart a slight horizontal motion to the axis of the telescope axle in addition to the desired vertical axis movement necessary to correct misalignment of telescope line of sight plumb. Telescope axle bearing mounting and adjustment means of the present invention provides a substantially true vertical shift in the telescope axle, thereby obviating compound misalignment inherent in earlier eccentric bearing adjustment arrangements.
While the requisite stability of a fine adjustment drive train was previously dependent upon close tolerance bearings and mechanical conjoint fittings, the drive mechanism in the present invention exhibits exceptional stability despite the fact that it employs a fixture bearing assembly of extremely loose tolerance. The kinematic balance achieved in this structure results in a surprising degree of firm, smooth, and repeatable fine adjustment operation. The lack of close tolerance elements and machining in this fixture bearing makes possible a great reduction in manufacturing and assembling costs and, in addition, yields a bearing which requires substantially no maintenance, yet resists wear over exceptionally long periods of use.